What is Pi?Pi (π) is a number that is roughly equal to 3.14159. Pi is the ratio of the circumference (perimeter) of any sized circle to its diameter. No matter what sized circle you have, the ratio of its circumference to its diameter is always the same -- pi. For example, when a circle's diameter is 1, its circumference is π. When a circle's diameter is 2, its circumference is 2 times π.

Pi is Irrational
Pi is an irrational number. This means that pi cannot be written as a fraction of two whole numbers and that the decimal digits of pi continue infinitely without any repeating pattern. Pi is roughly 3.14159265358979323846.... Using computers, pi has been calculated to many million of digits past the decimal point. For many purposes, using the approximation 3.1416 is sufficient. Good approximations for pi are 22/7 and 355/113.

Formulas
Pi is used in many calculations, including finding the circumference and area of a circle, and the volume and surface area of a sphere, cone, and cylinder. Pi is also used in geometry, probability and statistics, and many other fields.

Circumference

Area

Circle(radius = r)

2π r

π r2

Surface Area

Volume

Sphere(radius = r)

4π r2

(4/3) π r3

Cylinder
(radius = r, height = h)

2π r h (without the ends)

π r2 h

The Name Pi
Mathematicians began using word pi (the Greek letter π) for this ratio in the early 1700s; the name pi was given to this number because p is short for perimeter.

Pi Day
Pi Day is a mathematical holiday celebrated by math classes on March 14th (3/14), with the peak celebration occuring at at 1:59 p.m (3/14 1:59). Pie, pizza, and other round foods are often served at Pi Day celebrations.

Calculating PiA Printable WorksheetIn this worksheet, the student measures the circumference and diameter of three circles, and then calculates the ratios of the circumferences to the diameters (approximating pi). The student will need a ruler and a short length of string or yarn (a bit less than a foot long); the yarn is used to measure the circumferences. Or go to the pdf version.

What is Pi?A Printable WorksheetA printout about what pi is. Measure the circumference and diameter of four printed circles, then determine what relationship between the circumference and the diameter gives a constant value (pi). This printable worksheet demonstrates to the student that pi is the ratio of a circle's circumference to diameter, for any circle The student will need a ruler and a short length of string or yarn (a bit less than a foot long); the yarn is used to measure the circumferences. Or go to the pdf version.

Print a card that is good for many occasions, including Happy Birthday, Valentine's Day, Mother's Day, Father's Day, Congratulations, Graduation, Get Well Soon, Pi Day, Easter, Christmas, Hanukkah, Ramadan, Anniversary, New Year or Many Thanks. Just print the card, then check off the appropriate occasion.